A Flim map shows the wall and the tissue structure of a vein (Physics World)

Routine diagnosis of cancer in the not-too-distant future may simply involve firing a laser at suspect tissue and seeing how it reacts.

The tell-tale signs of malignancy would show up as a particular colour on a screen.

The technology that will allow this kind of "optical biopsy" is now in rapid development. It should be a lot easier on patients who currently have to go under the knife to have tissue removed for investigation.

The new style of test could be done on the end of a probe - perhaps an endoscope - pushed through a small incision or down the throat.

"It's basically about taking a histopathology lab to the patient rather than bringing a piece of the patient to the laboratory," says Professor Paul French from Imperial College London.

Rapid bursts

He is developing a technique known as fluorescence-lifetime imaging (Flim). This involves firing a laser at tissue in very short, rapid bursts and watching for the equally short consequences.

When laser light hits a sample, the molecules in it absorb the energy before emitting the light themselves at a different wavelength. In other words, the molecules fluoresce.

Flim relies on the fact that the time over which the light is emitted - the fluorescence lifetime - varies for different kinds of tissue or for tissue in a changed state. This is the key to using Flim to diagnose cancer.

Professor French explained how it would be done to the Institute of Physics 2000 Congress in Brighton. He told the BBC: "When something fluoresces, the standard thing you do is look at the intensity, and from that you get at the quantum efficiency, or you look at the wavelength, and from that you can tell which molecule it is.

"But you can also look at the time over which it fluoresces which is just as much a property of the molecule as the wavelength. It's kind of like using another dimension."

False colour map

The times involved, however, are extremely short - lifetimes that last just a few million-millionths of a second (picoseconds). "Basically not many people are doing this work because not many people have access to the ultra-fast laser technology that you would need to tell the difference between different molecules.

"We now have a system which can tell the difference between molecules that have a lifetime difference of only three picoseconds. The system does that in just a few minutes and produces an image.

"You'll find that tissues like collagen and elastin will show up with different lifetimes and can be represented on a false colour map."

The trick of course will be to develop technology that can recognise lifetimes that are signatures of disease. Any device would also need to be packaged in a way that made it practical - on the scale of an ultrasound scanner, for example.

This could then be used by pathology labs and even by surgeons who wanted to make sure they had removed all the cancerous tissue during an operation.